Toni Braxton’s “Un-Break My Heart” was a career-defining song for the R&B artist in 1996 — going platinum, winning a Grammy Award and selling more than 2.4 million copies.

The only problem? Braxton initially didn’t like it.

“I heard the song [and] I thought, ‘This is nice, it’s all right.’ [My record producers] said, ‘We think this is a smash,’” Braxton, 48, tells The Post.

“I knew I wanted to change my image and be a little sexier. It was strategic to be an [adult contemporary] artist because of the texture of my voice, but I wanted to be 25 years old. I thought ‘Un-Break My Heart’ would put me back in the same category of being an adult contemporary artist.”

“But they turned out to be right,” she adds. “I didn’t want to do it but I did it and it was the biggest thing in my career.”

That anecdote, and many others, are recounted in the Lifetime movie “Toni Braxton: Unbreak My Heart,” premiering Saturday at 8 p.m., which is based on the singer’s 2014 memoir. The biopic traces Braxton’s ascent from her strict Christian upbringing in Maryland — singing in church with her four younger sisters (who now star in their own WE tv reality show “Braxton Family Values”) — to R&B icon as the winner of seven Grammy Awards with 67 million records sold.

Starring as the young Toni is 24-year-old newcomer Lex Scott Davis. As executive producer, Braxton (who also narrates the movie) chose not to be involved in the casting process, but she did have final approval on who would portray her.

“When I met [Lex] I said, ‘She’s me.’ She stands like me, she plays piano like me, she holds her head like me, especially when I was younger,” Braxton says. “I was amazed that she had all my natural mannerisms.”

She stands like me, she plays piano like me, she holds her head like me … she had all my natural mannerisms.

- Toni Braxton on Lex Scott playing her in 'Unbreak My Heart.'

The movie delves into the singer’s personal life: Her friendship with longtime collaborator Kenneth “Babyface” Edmonds (who is also producer on the movie) and romance with Mint Condition keyboardist Keri Lewis, whom she very publicly divorced in 2013. There’s her younger son’s diagnosis of autism and her own battle with Lupus, which temporarily sidelined her career. (She’s currently in the studio working on a new album, hopefully out this fall.)

Also covered is Braxton’s two bankruptcies, including the fact that she won her first bankruptcy case against LaFace Records and Arista (she received $22 million), but a 10-year gag order had prevented her from speaking about it — or combatting the frivolous image of her promulgated in the press.

“I hated being seen as a walking cliché. ‘Oh, another artist lost her money,’ ” she says.